1. Field of the Invention
The invention relates to a process for the preparation of thermosettable urea-formaldehyde resins with a molar ratio of urea to formaldehyde of 1:1 to 1:1.5 and the use of such resins for the impregnating and/or coating of supporting webs used for the surface finishing of wood-based materials.
2. Description of the Prior Art
Thermosettable urea-formaldehyde resins are especially desirable as coating resins because of their comparatively low raw material costs. Nevertheless, melamine-formaldehyde resins are preferably used, especially for coatings of high quality. The reason for this lies mainly in the fact that the preparation of urea-formaldehyde resins of high quality presents great difficulties. Relative to melamine-formaldehyde resins, urea-formaldehyde resins have the particular disadvantage of decreased water resistance.
In recent years, however, processes for the preparation of high-grade, temperature stable, thermosettable urea-formaldehyde resins with outstanding surface-forming properties have become known. For example, German Offenlegungsschrift No. 24 48 472 describes the preparation of urea-formaldehyde resins for use in impregnating and coating of supporting webs and which, because of their special properties, make high quality surface coatings possible on wood-based panels.
However, this thermosettable urea-formaldehyde resin still possesses certain disadvantages. One of these problems is that, in addition to the resin having a molecular weight range desirable for the application, the resin solution also contains portions of resin of significantly lower and higher molecular weights than the desired range. The higher molecular portions of resin may frequently lead to precipitates in the solution and hinder the penetration of the resin into the coating web, which, as a rule, consists of paper. The lower molecular portions increase the hydrophilicity of the coating resin and, in the set state, readily lead to blocking (sticking together) of the coated supporting webs while they are being stored.
The thermosettable urea-formaldehyde resins are prepared in the usual manner in that, initially, urea and formaldehyde are reacted in the alkaline pH range. In this reaction, there is essentially only the addition of formaldehyde to the NH.sub.2 group of the urea with the formation of N-methylol groups. In a subsequent acidic reaction step, these initially obtained compounds are condensed to higher molecular, settable products. The course of the reaction and the synthesis of the reaction products formed depend essentially on the molar ratio of urea to formaldehyde which has been selected. Depending on the availability of formaldehyde, the products obtained are highly compatible with water, soluble to a very limited extent, or completely insoluble in water.
The limited water solubility of urea-formaldehyde resins with a low proportion of formaldehyde is the consequence of a very broad distribution of molecular weights. In the case of a decreased formaldehyde availability, the condensation in the acidic pH range takes place much less uniformly than in the case of a greater formaldehyde availability. This decreased formaldehyde availability leads to the formation of higher molecular, partially insoluble condensation products. At the same time, as a consequence of the irregular course of the reaction, relatively low molecular condensation products are also formed. As already mentioned, the low molecular resin portions particularly increase the water sensitivity of the coating resin, while the high molecular condensation products are not as soluble and prevent the penetration of the resin solutions into the supporting webs.
If the molar ratio of urea to formaldehyde is changed by using higher formaldehyde contents, the formation of higher molecular condensation product is suppressed or, under certain circumstances, is avoided completely. However, in the set state, the resins become increasingly more water sensitive. This impairs the end-use properties of the hardened resins.
U.S. Pat. No. 3,198,761 describes a process according to which urea-formaldehyde resins, with a molar ratio of urea to formaldehyde of 1:1 to 1:0.5, may be prepared.
SU Pat. No. 496,291 describes a process wherein an aminoplast adhesive resin is produced in a vacuum by successive condensations of urea and formaldehyde. The first is at a pH value of 7 to 8 and a temperature of 80.degree. to 95.degree. C., and then is at a pH value of 4 to 5 and a temperature of 80.degree. to 95.degree. C. The next condensation is with an additional amount of a nitrogenous compound, particularly urea, thiourea, melamine and cyanuric acid. This process is intended to ensure a low content of free formaldehyde in the finished resin and the waste waters and to reduce the amount of free formaldehyde which is separated during the hot bonding of materials. For this purpose, the precondensation is carried out at a pH value of 9 to 11 and a temperature of 65.degree. to 95.degree. C., wherein the ratio of the total amount of nitrogenous compound and formaldehyde is in the molar range of 1:0.9 to 1.35.
In the production of this resin, widely varying molecular weights are obtained. Particularly, higher molecular condensation products are formed which are, in part, insoluble. This can be recognized by the milky turbidity of the adhesive resin solution. Therefore, these resins are also unsuitable for impregnating and coating paper webs which are intended for the surface finishing of wood-based panels.
According to this process, urea and formaldehyde are initially reacted in the alkaline range, using a molar ratio of 1:1.3 to 1:1.66 and are subsequently condensed at a pH from 2.5 to 5.0. To the reaction product thus obtained, urea is added in an amount such that a molar ratio of urea to formaldehyde of 1:1 to 1:0.5 results, and the mixture is condensed once again in the pH range of 3.0 to 5.5.
By this procedure, resins of varying molecular weights are formed which contain, in addition to higher molecular condensation products, a large portion of very low molecular products as well as free urea. These resins are not suitable for impregnating and coating paper webs, which are intended for the surface finishing of wood-based panels.